The quasi resonant functioning mode of flyback DC--DC converters at steady state conditions is particularly efficient because, compared to traditional flyback applications (hard switching mode), it allows for a reduction of power dissipation during the switching phases and a reduction of electromagnetic noise.
FIG. 1 shows the basic scheme of a flyback converter for quasi resonant applications. The switching element Q1 is indicated as being a bipolar junction transistor though it may be of a different type. The D1 and C1 components allow for a quasi resonant functioning, also called QRC mode. In traditional applications, such as in hard-switching applications, their function is performed by dedicated snubber or clamper circuits.
The type of control of the switchings of the (Q1) power switch is similar to that of selfoscillating circuits, commonly named SOPS (Self Oscillating Power Supply), because the switch-on is commanded always in the vicinity of the instant at which the current on the secondary winding of the flyback transformer becomes null. Hence, the converter always functions in a discontinuous manner, that is, with the current becoming null at every cycle, though remaining at the border between continuous and discontinuous functioning conditions.
During the ON phase of Q1, the D2 diode is OFF and there is an accumulation of energy in the primary winding of the transformer, which is transferred to the secondary during the OFF phase of Q1. In this phase the voltage Vc on the Q1 terminals is EQU Vc=Valim+(N1:N2) V2 (being V2 Vout)
When the energy is completely transferred (I.sub.F(D2) =0), the voltage Vc oscillates at the resonant frequency given by ##EQU1##
By suitably sizing the electrical parameters it is possible to produce an oscillation capable of allowing the diode D1 to conduct for a short period of time in order to realize a control transistor of the Q1 power during this phase, thus eliminating of the switch-on losses.
Therefore, the flyback converter belongs to the class of the so-called "zero-voltage quasi resonant" converters. These converters are frequently used in TV and VCR power supplies, wherein the input voltage Valim is obtained by rectifying and filtering the main voltage. Such a preference is due also to the fact that the architecture of these converters allows for multiple outputs by simply increasing the number of secondary windings of the flyback transformer. The auxiliary winding AUS is used to self power the control circuit during steady state functioning.
For such applications, during the switch-off phase, the power transistor Q1 that implements the switch must withstand voltages that may reach or even exceed a thousand volts. In case of a completely monolithic realization (control circuits and power device realized on the same chip), a fabrication technology usually referred to as "Smart Power", suitable for high voltage applications, must be used.
Traditional QRC flyback converters are realized with discrete components, or in the form of an integrated device containing a low voltage control circuit, a high voltage power MOS transistor and eventually some of the passive components, later realized in so-called SMD technology. As depicted by way of an example in FIG. 3, the switch-on under a quasi resonant condition is obtained through an external Tdelay network connected to the auxiliary winding Aus and dimensioned so as to synchronize the condition Vc=0 with the switching of a comparator of the control circuit contained in the CONTROL-IC block, within the time interval indicated as Tdelay in the diagrams of FIG. 2.
Thus, the integrated circuit of the block CONTROL IC controls a selfoscillating or SOPS functioning mode of the converter. Replacing the delay block TDELAY with a resistor would produce a classic hard-switching flyback application.
In traditional circuits, the QRC function is thus obtained through external networks, which especially in TV applications, where there are large variations of either the supply voltage and the load, necessitate a substantial number of components, as illustrated in the detail of the block Tdelay of FIG. 3. However, other circuit arrangements of the delay network may be used, depending on the specific characteristics of the application.
Therefore, known circuits have the drawback of requiring the realization of a Tdelay network with discrete elements external to the integrated circuit. Moreover, the switching under zero voltage conditions is tied to the precision of the Tdelay network (that is, to the spread of the actual values of the network components) as well as to the electrical parameters that establish the resonance frequency (the spread of the values of Lp and Cr).